avcodec/vc1_block: Check get_vlc2() return before use

Fixes: index -1 out of bounds for type 'const uint8_t [185][2]'
Fixes: 15720/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MSS2_fuzzer-5666071933091840

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 2cb1f79735)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/vc1_block.c

@@ -507,6 +507,8 @@ static int vc1_decode_ac_coeff(VC1Context *v, int *last, int *skip,
         int escape = decode210(gb);
         if (escape != 2) {
             index = get_vlc2(gb, ff_vc1_ac_coeff_table[codingset].table, AC_VLC_BITS, 3);
+            if (index < 0)
+                return AVERROR_INVALIDDATA;
             run   = vc1_index_decode_table[codingset][index][0];
             level = vc1_index_decode_table[codingset][index][1];
             lst   = index >= vc1_last_decode_table[codingset];